A wireless star network includes a master node (master) and a set of slave nodes (slaves). The slaves transmit packets sequentially to the master using time division multiplexing, or simultaneously when frequency division multiplexing is used. If transmissions fail, then packets are retransmitted. It is desired to improve the performance of star networks.
One improvement uses temporal, spatial or frequency diversity, which results in different reception conditions. Orthogonal frequency division multiple access (OFDMA) provides reliable multipath channels, and high data rates using frequency and temporal diversity, see IEEE 802.16m (WiMAX), IEEE 802.22 and 3GPP LTE standards.
In OFDMA star networks, priority of traffic classes and urgency level of each packet can be used as criteria for frequency (channel) selection to achieve low latency for high priority traffic, without exploiting path diversity. Optimum subcarrier allocation in OFDMA networks over frequency selective slow fading channels has been described.
Path diversity in OFDMA networks uses multiple nodes as a collection of distributed antennas to improve reliability at the physical layer by obtaining a higher signal-to-noise (SNR) ratio. A distributed opportunistic access scheme for OFDMA with a back-off mechanism uses channel state information to avoid collisions. Spatial diversity can also be provided at the link layer.
Using half-duplex cooperation for OFDMA, pairs of cooperative nodes can transmit data sequentially within each OFDMA superframe by “piggy-backing” previously received packets to achieve path diversity. However, the pair of cooperating nodes must switch between transmit and receive states multiple times within a single OFDMA frame. In addition, unconditional relaying decreases efficiency when reliable channels are available.